Rigid-scope optical system, imaging apparatus, and endoscope system

ABSTRACT

A rigid-scope optical system includes: an image-formation optical system that causes an image in each of wavelength bands to be formed in a predetermined imaging device, the wavelength bands including a fluorescence wavelength band and a visible light wavelength band; and a color-separation-prism optical system having a dichroic film that separates an optical path of light to be imaged into an optical path of the visible light wavelength band and an optical path of the fluorescence wavelength band, in which the image-formation optical system causes the respective images to be formed in a fluorescence imaging device and a visible light imaging device, the fluorescence imaging device and the visible light imaging device being disposed to cause an amount of misalignment to correspond to a difference between an optical path length of fluorescence and an optical path length of visible light.

TECHNICAL FIELD

The present disclosure relates to a rigid-scope optical system, animaging apparatus, and an endoscope system.

BACKGROUND ART

In recent years, in the medical field, there has been an increasingdemand for not only observation of an affected area using lightbelonging to the visible light wavelength band but also observation ofan affected area using fluorescence belonging to the near-infraredwavelength band when performing a surgery using an endoscopy. This isbecause surgery using an ICG (indocyanine green) reagent, which emitsfluorescence having a wavelength of 830 to 840 nm by being irradiatedwith near-infrared excitation light having a wavelength of around 800nm, as a marker in the body for identifying a site, has begun to becomewidespread. The above-described ICG reagent is a safe reagent which isnot toxic even when injected into the body, and is particularly used forthe presence or absence of blood flow in brain surgery, identificationof cancer in a sentinel lymph node in breast cancer, etc., and clinicalresearch for endoscopic surgery is proceeding.

However, most of the fluorescent reagents used in the medical field,such as ICG and the like, have very low fluorescence efficiency, so thata highly sensitive camera is used to image a subject of interest (i.e.,a site emitting fluorescence). Because ICG-compatible endoscopic cameraheads and photographing systems that are available on the market todayuse, as imaging devices, existing visible light RGB single-plate orthree-plate sensors, a sensitivity in the near-infrared wavelength bandis not sufficient, and image quality and resolution are not comparableto those of a visible light picture image.

Further, the above-described sensor mainly utilizing R, G, and B is notable to image a visible light ray and a near-infrared ray at a time, andis only able to perform imaging in one of the wavelength bands.Therefore, it is not possible to compare the affected area identified bythe near-infrared ray (i.e., fluorescence) with a video of the visiblelight ray, and there has been a possibility that accuracy of the surgeryis decreased due to misalignment of the site caused by the switching.

For ensuring the accuracy of the surgery, there has been proposed animaging method called a time division method in which a picture imagecaptured under the visible light ray and a picture image captured underthe near-infrared ray are simultaneously displayed in a pseudo manner bysimultaneously and timely switching modes of a light source and aimaging device every frame. For example, PTL 1 proposes, for achievingimaging in the time division method described above, installing aspecial band-pass filter in a preceding stage of a visible light RGBsensor, and performing strict switching control between an imaging modeand a light source illumination mode of the sensor.

However, in a case of the technique borrowing an imaging device of thevisible light band described in PTL 1, a lens group in which chromaticaberration is corrected specialized for the visible light band is oftenused, and in such a case, a picture image of the near-infraredwavelength band inevitably becomes blurred due to chromatic aberration.In addition, in the system using the above-described time divisionformat, it is very difficult to focus on each frame by auto-focus everytime the switching is performed, and a picture image of one of thewavelength bands is inevitably simultaneously imaged with a poorresolution at all times.

For this reason, as a method of achieving simultaneous acquisition of apicture image of the visible light wavelength band and a picture imageof the near-infrared wavelength band in a method other than the timedivision method, a method has been proposed in which an optical path towhich light of an acquired image is guided is branched into an opticalpath for the visible light wavelength band and an optical path for thenear-infrared wavelength band, and then an imaging device for thevisible light and an imaging device for the near-infrared light are used(see, for example, PTL 2 and PTL 3).

CITATION LIST Patent Literature

PTL 1: Japanese Patent No. 6088629

PTL 2: Japanese Unexamined Patent Application Publication No. 2017-53890

PTL 3: Japanese Patent No. 6147455

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Here, considering endoscopic observations in the medical field, it isnecessary to further increase the resolution of a picture image to beacquired for further improving safety of a procedure performed by aphysician or the like. In the case of the technique of the time divisionmethod as disclosed in PTL 1: it is demanded to increase the resolutiononly by design change on the optical system to be used for furtherincreasing the resolution of the picture image, thereby inevitablyincreasing the number of lenses to be used; and it is necessary to use aglass material having a low anomalous dispersibility for causing thevisible light and the near-infrared light to be imaged simultaneously ata high resolution. However, many of the glass materials each having alow abnormal dispersibility are relatively fragile, and there is aconcern about reliability in the medical field in which processes suchas high temperature disinfection and chemical disinfection are performedat all times. Further, the size of the apparatus itself is increased ifthe number of lenses used is increased, which reduces the convenience ofa user.

In contrast, the methods as disclosed in PTL 2 and PTL 3 are able tomaintain reliability of the apparatus and to reduce the size of theapparatus. However, in the method disclosed in PTL 2, for example, nostudies have been carried out on an optical system for achieving furtherhigher resolution, such as a 4K resolution. Further, in the opticalsystem disclosed in PTL 3, a size of a prism for branching the opticalpath is too small, which raises a concern that a flare of an internalreflection of the prism will increase, and it has been difficult toachieve higher resolution.

As described above, there is a current demand for a medical-use imagingapparatus that is able to achieve reduction in size while ensuringreliability as an apparatus, and to obtain a captured picture imagehaving a more excellent resolution, such as a 4K resolution, forexample.

Accordingly, in view of the above circumstances, the present disclosureproposes a rigid-scope optical system, an imaging apparatus, and anendoscope system that are able to achieve reduction in size whileensuring reliability as an apparatus, and to achieve further resolutionof a captured picture image to be obtained.

Means for Solving the Problems

According to the present disclosure, there is provided a rigid-scopeoptical system including: an image-formation optical system that causesan image in each of wavelength bands to be formed in a predeterminedimaging device, the wavelength bands including a fluorescence wavelengthband belonging to a near-infrared light wavelength band and a visiblelight wavelength band; and a color-separation-prism optical systemhaving a dichroic film that separates an optical path of light to beimaged by the image-formation optical system into an optical path of thevisible light wavelength band and an optical path of the fluorescencewavelength band, in which the image-formation optical system causes therespective images to be formed in a fluorescence imaging device and avisible light imaging device, the fluorescence imaging device and thevisible light imaging device being disposed to cause an amount ofmisalignment between a fluorescence image formation position and avisible light image formation position caused by the image-formationoptical system to correspond to a difference between an optical pathlength of fluorescence and an optical path length of visible light, thefluorescence and the visible light forming the respective images via thecolor-separation-prism optical system, and, where a focal length of theimage-formation optical system is represented by f [mm], and anair-equivalent optical path length from the image-formation opticalsystem to an imaging device is represented by Fb [mm], theimage-formation optical system has the focal length and theair-equivalent optical path length that satisfy a condition representedby the following expression (1),

Fb/f>0.72   expression (1).

Further, according to the present disclosure, there is provided animaging apparatus including a rigid-scope optical system, therigid-scope optical system including an image-formation optical systemthat causes an image in each of wavelength bands to be formed in apredetermined imaging device, the wavelength bands including afluorescence wavelength band belonging to a near-infrared lightwavelength band and a visible light wavelength band, acolor-separation-prism optical system having a dichroic film thatseparates an optical path of light to be imaged by the image-formationoptical system into an optical path of the visible light wavelength bandand an optical path of the fluorescence wavelength band, a visible lightimaging device that forms an image of the visible light wavelength band,and a fluorescence imaging device that forms an image in thefluorescence wavelength band, in which the visible light imaging deviceand the fluorescence imaging device are disposed to cause an opticalpath difference between an optical path length of a visible lightwavelength band and an optical path length of a fluorescence wavelengthband to correspond to an amount of misalignment between a fluorescenceimage formation position and a visible light image formation positioncaused by the image-formation optical system, the visible light formingan image in the visible light imaging device via thecolor-separation-prism optical system, the fluorescence forming an imagein the fluorescence imaging device via the color-separation-prismoptical system, and, where a focal length of the image-formation opticalsystem is represented by f [mm], and an air-equivalent optical pathlength from the image-formation optical system to an imaging device isrepresented by Fb [mm], the image-formation optical system has the focallength and the air-equivalent optical path length that satisfy acondition represented by the following expression (1),

Fb/f>0.72   expression (1).

Further, according to the present disclosure, there is provided anendoscope unit including: a rigid-scope unit that generates an image ofa predetermined imaging target of a fluorescence wavelength bandbelonging to a near-infrared light wavelength band and an image of thepredetermined imaging target of a visible light wavelength band; animaging unit that includes a rigid-scope optical system coupled to therigid-scope unit, a visible light imaging device in which the image ofthe visible light wavelength band is formed, and a fluorescence imagingdevice in which the image of the fluorescence wavelength band is formed,and generates a captured picture image of the imaging target of thefluorescence wavelength band and a captured picture image of the imagingtarget of the visible light wavelength band, in which the rigid-scopeoptical system includes an image-formation optical system that causes animage in each of the fluorescence wavelength band and the visible lightwavelength band to be formed in a predetermined imaging device, and acolor-separation-prism optical system having a dichroic film thatseparates an optical path of light to be imaged by the image-formationoptical system into an optical path of the visible light wavelength bandand an optical path of the fluorescence wavelength band, the visiblelight imaging device and the fluorescence imaging device are disposed tocause an optical path difference between an optical path length of avisible light wavelength band and an optical path length of afluorescence wavelength band to correspond to an amount of misalignmentbetween a fluorescence image formation position and a visible lightimage formation position caused by the image-formation optical system,the visible light forming an image in the visible light imaging devicevia the color-separation-prism optical system, the fluorescence formingan image in the fluorescence imaging device via thecolor-separation-prism optical system, and, where a focal length of theimage-formation optical system is represented by f [mm], and anair-equivalent optical path length from the image-formation opticalsystem to an imaging device is represented by Fb [mm], theimage-formation optical system has the focal length and theair-equivalent optical path length that satisfy a condition representedby the following expression (1),

Fb/f>0.72   expression (1).

According to the present disclosure, the image-formation optical systemcauses the respective images to be formed in the fluorescence imagingdevice and the visible light imaging device, the fluorescence imagingdevice and the visible light imaging device being disposed to cause theamount of misalignment between the fluorescence image formation positionand the visible light image formation position caused by theimage-formation optical system to be in association with the differencebetween the optical path length of the fluorescence and the optical pathlength of the visible light, the fluorescence and the visible lightforming the respective images via the color-separation-prism opticalsystem, and the image-formation optical system satisfies the conditionrepresented by the expression (1)

Effects of the Invention

As described above, according to the present disclosure, it is possibleto achieve reduction in size while ensuring reliability as an apparatus,and to achieve further resolution of a captured picture image to beobtained.

It is to be noted that the effects described above are not necessarilylimitative. With or in the place of the above effects, there may beachieved any one of the effects described in this specification or othereffects that may be grasped from this specification.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an explanatory diagram schematically illustrating an overallconfiguration of a rigid-scope optical system according to an embodimentof the present disclosure.

FIG. 2A is an explanatory diagram schematically illustrating acolor-separating-prism optical system included in the rigid-scopeoptical system according to the embodiment.

FIG. 2B is an explanatory diagram schematically illustrating thecolor-separating-prism optical system included in the rigid-scopeoptical system according to the embodiment.

FIG. 3 is an explanatory diagram schematically illustrating thecolor-separating-prism optical system included in the rigid-scopeoptical system according to the embodiment.

FIG. 4 is an explanatory diagram schematically illustrating aconfiguration of an image-formation optical system included in therigid-scope optical system according to the embodiment.

FIG. 5 is an explanatory diagram schematically illustrating an overallconfiguration of an endoscope system according to the embodiment.

FIG. 6 is a block diagram illustrating an example of a hardwareconfiguration of a CCU included in the endoscope system according to theembodiment.

FIG. 7A is a schematic view of a configuration of an image-formationoptical system of Example 1.

FIG. 7B is a schematic view of a configuration of an image-formationoptical system of Example 2.

FIG. 7C is a schematic view of a configuration of an image-formationoptical system of Example 3.

FIG. 7D is a schematic view of a configuration of an image-formationoptical system of Example 4.

FIG. 7E is a schematic view of a configuration of an image-formationoptical system of Example 5.

FIG. 8A is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 1.

FIG. 8B is a graph indicating a simulation result of lateral aberrationof the image-formation optical system of Example 1.

FIG. 8C is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 1.

FIG. 9A is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 2.

FIG. 9B is a graph indicating a simulation result of lateral aberrationof the image-formation optical system of Example 2.

FIG. 9C is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 2.

FIG. 10A is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 3.

FIG. 10B is a graph indicating a simulation result of lateral aberrationof the image-formation optical system of Example 3.

FIG. 10C is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 3.

FIG. 11A is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 4.

FIG. 11B is a graph indicating a simulation result of lateral aberrationof the image-formation optical system of Example 4.

FIG. 11C is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 4.

FIG. 12A is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 5.

FIG. 12B is a graph indicating a simulation result of lateral aberrationof the image-formation optical system of Example 5.

FIG. 12C is a graph indicating a simulation result of longitudinalaberration of the image-formation optical system of Example 5.

MODES FOR CARRYING OUT THE INVENTION

The following describes a preferred embodiment of the present disclosurein detail with reference to the accompanying drawings. It is to be notedthat, in this description and the accompanying drawings, components thathave substantially the same functional configuration are indicated bythe same reference signs, and thus redundant description thereof isomitted.

It is to be noted that description is given in the following order.

-   1. Embodiment    -   1.1. Regarding Rigid-Scope Optical System    -   1.2. Regarding Imaging Apparatus    -   1.3. Regarding Endoscope System-   2. Examples

EMBODIMENT [Regarding Rigid-Scope Optical System]

First, referring to FIGS. 1 to 4, a rigid-scope optical system accordingto a first embodiment of the present disclosure will be described indetail. FIG. 1 is an explanatory diagram schematically illustrating anoverall configuration of the rigid-scope optical system according to thepresent embodiment. FIGS. 2A to 3 are each an explanatory diagramschematically illustrating a color-separating-prism optical systemincluded in the rigid-scope optical system according to the presentembodiment. FIG. 4 is an explanatory diagram schematically illustratinga configuration of an image-formation optical system included in therigid-scope optical system according to the present embodiment.

The rigid-scope optical system according to the present embodiment is anoptical system used for imaging an image obtained by a rigid scope, alsocalled a rigid endoscope. First, referring to FIG. 1, an overallconfiguration of a rigid-scope optical system 1 according to the presentembodiment will be briefly described below.

[Regarding Overall Configuration]

The rigid-scope optical system 1 according to the present embodimentincludes, as schematically illustrated in FIG. 1, an image-formationoptical system 10, and a color-separation-prism optical system 20.Further, outside the rigid-scope optical system 1, a visible lightimaging device 3 and a fluorescence imaging device 4 are provided.

The visible light imaging device 3 is provided on an optical path of avisible light wavelength band branched by the color-separation-prismoptical system 20, and forms an image including light belonging to thevisible light wavelength band, out of an image of an imaging target. Thefluorescence imaging device 4 is provided on an optical path of afluorescence wavelength band branched by the color-separation-prismoptical system 20, and an image including light belonging to thefluorescence wavelength band is formed, out of the image of the imagingtarget. The visible light imaging device 3 and the fluorescence imagingdevice 4 are not particularly limited, and various known CCD sensors,CMOS sensors, and the like may be used therefor.

For example, the visible light imaging device 3 is preferably a CMOSsensor or a CCD sensor using a Bayer array color filter or another colorfilter having high color reproducibility of visible light. Further, thefluorescence imaging device 4 is preferably a CMOS sensor or a CCDsensor which does not use a color filter, for example, because it ispreferable to use the maximum sensitivity of the sensor fornear-infrared ray imaging.

The image-formation optical system 10 is an optical system for causingimages of the imaging target (specifically, an image in the visiblelight wavelength band and an image in the fluorescence wavelength bandbelonging to the near-infrared wavelength band) to be each formed in thecorresponding imaging device. The image-formation optical system 10according to the present embodiment has a lens configuration asdescribed in detail below, thereby being corrected in axial chromaticaberration and exhibiting an excellent optical characteristic in each ofthe fluorescence wavelength band and the visible light wavelength band.Such an image-formation optical system 10 is described in more detailbelow.

The color-separation-prism optical system 20 is an optical system thatseparates an optical path of light to be image by the image-formationoptical system 10 into an optical path of the visible light wavelengthband and an optical path of the fluorescence wavelength band. Thecolor-separation-prism optical system 20 according to the presentembodiment includes at least a dichroic film that separates light intolight belonging to the visible light wavelength band and light belongingto the fluorescence wavelength band, and such a dichroic film separatesthe optical path of the light to be imaged by the image-formationoptical system 10 into the two optical paths. Such acolor-separation-prism optical system 20 is also described in detailbelow.

[Regarding Color-Separation-Prism Optical System 20]

Next, the color-separation-prism optical system 20 according to thepresent embodiment will be described referring to FIGS. 2A to 3.

The color-separation-prism optical system 20 according to the presentembodiment is an optical system incorporated in the rigid-scope opticalsystem 1 according to the present embodiment for satisfying a desiredspecification.

For simultaneously imaging a visible light ray and a near-infrared ray(fluorescence) with high image quality (e.g., 4K resolution or higher),it is important to spectrally separate light into the visible light rayand the near-infrared ray and perform imaging with respective individualimaging devices corresponding to the visible light ray and thenear-infrared ray. Therefore, the rigid-scope optical system 1 accordingto the present embodiment is provided with a prism optical systemincluding a predetermined dichroic film as the color-separation-prismoptical system 20, thereby separating the optical path of the light tobe imaged by the image-formation optical system 10 into the optical pathof the visible light wavelength band and the optical path of thefluorescence wavelength band.

Such a color-separation-prism optical system 20 includes, for example,as schematically illustrated in the FIG. 2A, at least a color-separationprism 201, and a dichroic film 203 having a predetermined opticalcharacteristic is provided inside the color-separation prism 201. Thedichroic film 203 has an optical characteristic in which, for example,in a case where the imaging devices are disposed as illustrated in FIG.2A, light belonging to the visible light wavelength band is reflectedand light belonging to the near-infrared wavelength band (fluorescencewavelength band) is transmitted. In contrast, in a case where thepositions of the visible light imaging device 3 and the fluorescenceimaging device 4 are inverse to those illustrated in FIG. 2A, a dichroicfilm 203 having an optical characteristic in which the light belongingto the visible light wavelength band is transmitted and the lightbelonging to the near-infrared wavelength band (fluorescence wavelengthband) is reflected is used.

The light to be imaged by the image-formation optical system 10 entersthe color-separation prism 201 including the dichroic film 203 asdescribed above, and thus, the optical path of the light is branchedinto an optical path OP1 of the visible light wavelength band and anoptical path OP2 of the fluorescence wavelength band.

Further, an infrared cut filter or the like (not illustrated) ispreferably provided on an optical axis of the optical path OP1 of thevisible light wavelength band, for removing near-infrared light(fluorescence) that may leak into the optical path OP1 of the visiblelight wavelength band. The provision of such an infrared cut filtermakes it possible to further improve color reproducibility of a visiblelight picture image generated by the visible light imaging device 3.Similarly, a narrow-band bandpass filter (not illustrated) thattransmits light in the fluorescence wavelength band of interest ispreferably provided on an optical axis of the optical path OP2 of thefluorescence wavelength band, for removing excitation light and visiblelight that may leak into the optical path OP2 of the fluorescencewavelength band. This makes it possible to further improve contrast of afluorescence picture image generated by the fluorescence imaging device4.

Further, in the image-formation optical system 10 according to thepresent embodiment, imaging positions of the visible light ray and thenear-infrared ray are different from each other due to axial chromaticaberration. Thus, in FIG. 2A, for example, even if the fluorescenceimaging device 4 is provided at a position optically conjugate with thevisible light imaging device 3, a situation arises in which the image isin focus in the visible light imaging device 3 while the image is not infocus in the fluorescence imaging device 4. However, the rigid-scopeoptical system 1 according to the present embodiment is able to easilycorrect such axial chromatic aberration by branching the optical pathinto two by the color-separation-prism optical system 20 and providingtwo types of imaging devices.

For example, as schematically illustrated in FIG. 2A, an amount of focusmisalignment between the visible light ray and the near-infrared ray isgrasped in advance, and the visible light imaging device 3 is fixed inadvance at a position where the visible light ray is in focus. Then, thefluorescence imaging device 4 may be fixed and installed at a positionseparated from the position conjugate with the visible light imagingdevice 3 by a separation distance Δ, so that the identified amount ofmisalignment corresponds to an optical path difference between anoptical path length of the optical path OP1 of the visible lightwavelength band and an optical path length of the optical path OP2 ofthe fluorescence wavelength band.

Further, for example, as schematically illustrated in FIG. 2B, thevisible light imaging device 3 is fixed in advance at a position wherethe visible light ray is in focus, and the fluorescence imaging device 4is mounted to allow the installation position to be variable. Then, afluorescence picture image focusing mechanism 30 such as an actuator isprovided, which varies a separation distance Δ between: thecolor-separation-prism optical system 20 and the visible light imagingdevice 3; and the fluorescence imaging device 4. In such a case, thefluorescence picture image focusing mechanism 30 varies a relativepositional relationship between: the fluorescence imaging device 4; andthe color-separation-prism optical system 20 and the visible lightimaging device 3, thereby making it possible to easily correct the axialchromatic aberration caused by the image-formation optical system 10.

It is to be noted that, according to the method illustrated in FIG. 2B,for example, even in a case where a fluorescence emission position ofthe ICG reagent introduced into the inside of a viscera exists on backside of a surface of the viscera, it is possible to make a situation inwhich the surface of the viscera is observed in the visible light rayand the fluorescence emission position of the inside of the viscera isobserved in the near-infrared ray.

As the color-separation-prism optical system 20 having a function asdescribed above, it is preferable to use the color-separation-prismoptical system 20 like the one disclosed in PTL 2, for example, which isschematically illustrated in FIG.3.

In the color-separation-prism optical system 20 illustrated in FIG. 3,the color-separation prism 201 is a prism in which a first prism 211 anda second prism 213 are bonded to each other, and the first prism 211 andthe second prism 213 are bonded to each other via the dichroic film 203.That is, the dichroic film 203 is provided at an interface between thefirst prism 211 and the second prism 213.

The first prism 211 is a prism that functions as an optical path of thevisible light wavelength band through which light belonging to thevisible light wavelength band and light belonging to the fluorescencewavelength band (i.e., entering light) enter and the light belonging tothe visible light wavelength band is guided. Further, the second prism213 is a prism which functions as an optical path of the fluorescencewavelength band through which the light belonging to the fluorescencewavelength band is guided.

The light entering the first prism 211 travels straight in the firstprism 211, and the light belonging to the visible light wavelength bandand the light belonging to the fluorescence wavelength band areseparated from each other by the dichroic film 203 obliquely provided onthe optical axis.

The light belonging to the visible light wavelength band is reflected bythe dichroic film 203 and guided inside the first prism 211. Here, thereflected and separated light belonging to the visible light wavelengthband (i.e., the visible light ray) is totally reflected once at aposition A illustrated in FIG. 3, and is transmitted to the outside ofthe first prism 211. Thus, it is possible to make the angle of adichroic film 203-formed surface with respect to the optical axis closeto perpendicular. Conversely, the angle at which dichroic film 203according to the present embodiment is installed on the optical axis isset to cause a total reflection condition of the visible light ray atthe position A to be satisfied. The placement of the dichroic film 203in this way makes it possible to suppress variation in a spectralcharacteristic of the dichroic film 203 due to difference in an angle ofincidence between an upper light beam and a lower light beam, even in acase where light beams each having a bright F-value enter the firstprism 211, and to accurately perform wavelength separation.

The visible light ray transmitted through the first prism 211 is guidedto the visible light imaging device 3. In this case, an infrared cutfilter 217 may be provided between an exit surface of the first prism211 and the visible light imaging device 3. As such an infrared cutfilter 113, for example, it is possible to use a known absorbing filteror the like such as C5000 manufactured by HOYA Corporation.

In contrast, the light belonging to the fluorescence wavelength bandtransmitted through the dichroic film 203 enters the second prism 213and travels straight inside the second prism 213. An end surface of thesecond prism 213 on side opposite to side on which the dichroic film 203is provided (in other words, an exit surface of the second prism 213 ondownstream side of the optical axis) to be perpendicular to the opticalaxis, and the light belonging to the fluorescence wavelength band istransmitted to the outside of the second prism 213 while maintaining astate of being perpendicular to the exit surface of the second prism213.

The light belonging to the fluorescence wavelength band transmittedthrough the second prism 213 enters a narrow-band bandpass filter 215provided at a subsequent stage.

The preferred mode as color-separation prism 201 according to thepresent embodiment has been described above in detail. It is to be notedthat a material of the color-separation prism 201 according to thepresent embodiment is not particularly limited, and it is possible touse appropriately known optical glass or optical crystal depending on awavelength of light guided inside the color-separation prism 201.

Here, the respective optical characteristics of the dichroic film 203and the narrow-band bandpass filter 215 as illustrated in FIG. 3 will bespecifically described below focusing on the use of ICG as a fluorescentreagent.

The ICG has an excitation wavelength of approximately 769 nm, and whenthe ICG is excited by excitation light having such an excitationwavelength, fluorescent light belonging to the near-infrared wavelengthband having a wavelength of 832 nm is generated, for example.

In such a case, if the placement of the imaging device as illustrated inFIG. 3 is achieved, it is preferable that the optical characteristic(specifically, a spectral transmittance) of the dichroic film 203 have atransmittance of 90% or more in a wavelength band of 780 nm to 880 nm,and have a transmittance of 10% or less in a wavelength band of 400 nmto 720 nm.

In a case where the transmittance is less than 90% in the wavelengthband of 780 nm to 880 nm, a percentage of the fluorescence that is notable to transmit the dichroic film 203 increases and the brightness ofthe fluorescence picture image decreases, which is not preferable. Inaddition, in such a case, the fluorescence leaks into the visible lightimaging device 3, which lowers the contrast of the visible light pictureimage, which is not preferable from a viewpoint of image quality of thevisible light picture image.

Further, in a case where the transmittance exceeds 10% in the wavelengthband of 400 nm to 720 nm, a percentage of the visible light which is notreflected by the dichroic film 203 and is transmitted is increased, andthe brightness of the visible light picture image is lowered, which isnot preferable. In addition, in such a case, the visible light leaksinto the fluorescence imaging device 4, which lowers the contrast of thefluorescence picture image, which is not preferable from a viewpoint ofimage quality of the fluorescence picture image.

As is clear from the above explanation, the dichroic film 203 accordingto the present embodiment separates the entering light into two colors:light belonging to a predetermined fluorescence wavelength band a bandof a longer wavelength band than the predetermined fluorescencewavelength band; and light belonging to a shorter wavelength band than apredetermined fluorescence wavelength band. For example, the dichroicfilm 203 having the above-described spectral transmittance is a filmthat functions as a low-pass filter that separates the entering lightinto two groups with a boundary of 750 nm, which serves as a boundarybetween the visible light wavelength band and the fluorescencewavelength band as a boundary.

For example, the optical characteristic (spectral characteristic) of thedichroic film 203 as described above is relatively broad, and when thedichroic film 203 is achieved as an optical multilayered film, it ispossible to suppress the number of film layers to approximately severaltens of layers, and also to use a common vacuum evaporation method as amanufacturing method.

Further, it is also important that the narrow-band bandpass filter 215provided in the preceding stage of the fluorescence imaging device 4 bea filter having a bandpass property that reflects light in a wavelengthband other than the fluorescence wavelength band and transmits onlylight in the fluorescence wavelength band light.

In a case of focusing on the fluorescence emitted from ICG belonging tothe near-infrared band of a wavelength of 832 nm, it is preferable thatthe spectral transmittance of the narrow-band bandpass filter 215 have atransmittance of 90% or more in a wavelength band of 820 nm to 850 nm,and a transmittance of 10% or less in a wavelength band of 400 nm to 805nm and in a wavelength band of 860 nm to 1000 nm.

In a case where the transmittance is less than 90% in the wavelengthband of 820 nm to 850 nm, the percentage of the fluorescence thattransmits through the narrow-band bandpass filter 215 is reduced, andthis decreases the brightness of the fluorescence picture image, whichis not preferable. Further, in a case where the transmittance exceeds10% in the wavelength band of 400 nm to 805 nm and the wavelength bandof 860 nm to 1000 nm, external light other than the fluorescence, suchas excitation light having a wavelength around 800 nm, is reflected onthe fluorescence imaging device 4, and the contrast of the fluorescencepicture image is remarkably lowered, which is not preferable.

Further, if the wavelength band of the light transmitted by thenarrow-band bandpass filter 215 is wider than the wavelength band from820 nm to 850 nm, the near-infrared wavelength band contributing toforming the fluorescence picture image becomes too wide. As a result,even if it is possible to correct the center of gravity of axialchromatic aberration by a separation distance A, which will be describedlater, components having longer wavelength makes the image blurred andthe contrast is lowered, which is not preferable.

In addition, if the wavelength band of the light transmitted by thenarrow-band bandpass filter 215 is narrower than the wavelength bandfrom 820 nm to 850 nm, the light transmitted through the narrow-bandbandpass filter 215 approaches a monochromatic color, which enhanceseffects of correction of the axial chromatic aberration by theseparation distance Δ, which will be described later, but lowersbrightness of the fluorescence picture image, which is not preferable.

The narrow-band bandpass filter 215 according to the present embodimentis manufacturable by using a known optical material depending on thewavelength of the fluorescence of interest. For example, the narrow-bandbandpass filter 215 may be manufactured by forming an optical multilayerfilm on a glass substrate corresponding to BK7, or may be manufacturedby forming an optical multilayer film on such a substrate using avisible absorption glass such as R80 manufactured by HOYA Corporation asa substrate. This makes it possible to suppress transmittance of avisible light region and to contribute to improvement in the contrast ofthe fluorescence picture image, compared to a structure using the glasssubstrate.

It is to be noted that such a narrow-band bandpass filter 215 isformable by a vacuum evaporation method similarly to the dichroic film203, but has a spectral characteristic of a narrow band and a sharprising and falling shape; therefore, the number of film layers is largerthan the number of film layers of the dichroic film 203, and isapproximately several hundreds of layers. For this reason, it ispreferable to employ a film forming method such as an ion beamsputtering method that ensures high reliability than the vacuumevaporation method.

Further, the narrow-band bandpass filter 215 is preferably provided onthe optical path OP2 of the fluorescence wavelength band so as to havean entrance surface perpendicular to the optical axis. This makes itpossible to suppress variation in the spectral characteristic due todifference in an angle of incidence between an upper light beam and alower light beam, even in a case where light beams having a brightF-value have entered.

The color-separation-prism optical system 20 according to the presentembodiment has been described above in detail by referring to FIGS. 3 to2A.

[Regarding Image-Formation Optical System 10]

Next, referring to FIG. 4, the image-formation optical system 10according to the present embodiment will be described in detail.

The image-formation optical system 10 according to the presentembodiment is, as described above, an optical system for causing animage in each of the wavelength bands of the fluorescence wavelengthband belonging to the near-infrared light wavelength band and thevisible light wavelength band to be formed in a predetermined imagingdevice. The image-formation optical system 10 according to the presentembodiment has a lens design that satisfies a condition described indetail below, for example; thus, it is possible to achieve a pictureimage having an extremely excellent resolution higher than or equal to4K resolution, and it is also possible to achieve reduction in size ofthe apparatus itself by achieving reduction in size of the opticalsystem itself In addition, the optical system satisfying the conditiondescribed in detail below is configurable using a common glass materialhaving excellent temperature resistance and chemical resistance, whichis highly reliable in application to the medical field; thus, it ispossible to achieve the optical system having high reliability even inapplication to the medical field.

Where a focal length of the image-formation optical system 10 isrepresented by f [mm] and an air-equivalent optical path length from theimage-formation optical system to an imaging device is represented by Fb[mm], the image-formation optical system 10 according to the presentembodiment has the focal length f and the air-equivalent optical pathlength Fb that satisfy a condition represented by the followingexpression (101).

Fb/f>0.72   Expression (101)

The condition represented by the above expression (101) is a conditionalexpression related to a back focus of the image-formation optical system10, taking into account: the image-formation optical system 10; and asize of the color-separation prism 201 (e.g., the color-separation prism201 in which two prisms are bonded as illustrated in FIG. 3) located atthe subsequent stage of the image-formation optical system 10.

In a case where a value represented by Fb/f is 0.72 or less, it is notpossible to secure a physical space in which the color-separation prism201 is installed, taking into account an internal reflection of theprism and the like, which prevents the rigid-scope optical system 1 fromachieving high resolution. Further, although an upper limit of the valuerepresented by Fb/f is not specifically defined, if the valuerepresented by Fb/f becomes too large, an outer diameter of the rearmostlens becomes too large and an effective diameter of prism block frontside has to be increased. This does not contribute to reduction in size,which is not preferable. From the viewpoint of reduction in size of therigid-scope optical system 1, it is preferable that the upper limit ofFb/f is 1.00, for example. The value represented by Fb/f is morepreferably 0.75 or more and 1.00 or less, and still more preferably 0.80or more and 0.96 or less.

It is to be noted that, in the rigid-scope optical system 1 according tothe present embodiment, two types of air-equivalent optical path lengthsare considered, an air-equivalent optical path length to the visiblelight imaging device 3 and an air-equivalent optical path length to thefluorescence imaging device 4; however, the air-equivalent optical pathlength to the fluorescence imaging device 4 can be regarded as theair-equivalent optical path length to the visible light imaging device3+an amount of position misalignment, thus, in the above expression(101), the air-equivalent optical path length to the visible lightimaging device 3 may be used as Fb.

It is preferable that the image-formation optical system 10 satisfyingthe condition related to the back focus as described above include, inorder from object side to image side, at least a diaphragm 101, a firstlens group 103 having a positive refractive power, and a second lensgroup 105 having a positive refractive power, as schematicallyillustrated in FIG. 4, for example.

Hereinafter, although the term “n-th lens group” is frequently used inthis description, the lens group handled by such a term includes notonly a case where the lens group includes a set of two or more lensesbut also a case where the lens group includes one lens. Further, eachlens group may include various spherical lenses, may include variousaspherical lenses, or may include a combination of spherical lens(es)and aspherical lens(es).

Here, the first lens group 103 preferably includes, in order from theobject side to the image side, a lens having a negative refractive powerwith a concave surface facing the object side, and at least one lenshaving a positive refractive power. The second lens group 105 ispreferably a focus group that performs focusing depending on an objectdistance.

In a case where the first lens group 103 does not have the lens havingthe negative refractive power with the concave surface facing the objectside on the most object side, it becomes difficult to havetelecentricity of the optical system, and it is not possible to achievelong Fb satisfying the above expression (101). Further, in a case wherethe first lens group 103 is not a combination of the lens having thenegative refractive power and the at least one lens having the positiverefractive power, it is not possible to achieve a lens group having apositive refractive power in the first lens group 103 as a whole.

In addition, in a case where the second lens group 105 is not a lensgroup having the positive refractive power as a whole, it is notpossible to properly correct coma aberration, and thus, it is notpossible to achieve a better resolution, and in a case where the secondlens group 105 is not the focus group, the entire optical system has bemoved, which makes it difficult to suppress variation in the angle ofview.

In this case, it is preferable that the image-formation optical system10 according to the present embodiment satisfies a condition representedby the following expression (102), where an air-equivalent optical pathlength from the image-formation optical system 10 to thecolor-separation-prism optical system 20 is represented by L [mm].

1.4<L/f<1.8   Expression (102)

The conditional expression represented by the expression (102) is aconditional expression defining a total length of the image-formationoptical system 10. In a case where a value represented by L/f is 1.4 orless, the lenses have to be thinned or the like to secure a size of thecolor-separation prism 201 located at the subsequent stage of theimage-formation optical system 10; thus, not only the manufacturabilityis lowered, but also it may be difficult to reduce the size of theimage-formation optical system 10 while maintaining reliability in themedical field. Further, in a case where it is necessary to reduce thenumber of lenses, it may be difficult to reduce the size of theimage-formation optical system 10 while demanding for high resolution.In contrast, in a case where the value represented by L/f is 1.8 ormore, the total length of the image-formation optical system 10 becomestoo long, and it may be difficult to reduce the size of the rigid-scopeoptical system 1. The value represented by L/f is more preferably 1.5 ormore and 1.7 or less, and still more preferably 1.55 or more and 1.65 orless.

Further, in the image-formation optical system 10 according to thepresent embodiment, it is preferable that the second lens group 105further satisfies a condition represented by the following expression(103), where a focal length of the second lens group 105 is representedby f2 [mm].

1.0<f2/f<1.4   Expression (103)

The conditional expression represented by the expression (103) is aconditional expression defining the focal length f2 of a focus lensgroup achieved by the second lens group 105. In a case where a valuerepresented by f2/f is 1.0 or less, it is possible to shorten a focusstroke, but aberration correction may become unbalanced, which is notpreferable from the viewpoint of an optical characteristic achieved bythe image-formation optical system 10. In contrast, in a case where thevalue represented by f2/f is 1.4 or more, the focus stroke may becometoo large, and it may become difficult to reduce the size of theimage-formation optical system 10. The value represented by f2/f is morepreferably 1.15 or more and 1.35 or less, and still more preferably 1.2or more and 1.3 or less.

Further, as schematically illustrated in FIG. 4, the image-formationoptical system 10 according to the present embodiment preferably furtherincludes, between the diaphragm 101 and the first lens group 103, inorder from the object side to the image side, at least one of a thirdlens group 107 having a positive refractive power or a fourth lens group109 having a negative refractive power.

In such a case, it is preferable that the fourth lens group 109satisfies a condition represented by the following expression (104),where a focal length of the fourth lens group 109 in the image-formationoptical system 10 is represented by f4 [mm].

−0.80<f4/f<−0.35   Expression (104)

The conditional expression represented by the expression (104) is aconditional expression defining a ratio of a power of a concave lensincluded in the fourth lens group 109 provided for achieving a negativerefractive power to an entire power of the image-formation opticalsystem 10. In a case where a value represented by f4/f is −0.35 or more,the power of the concave lens included in the fourth lens group 109 maybecome too strong, and it may become difficult to achieve the placementof the lens groups as illustrated in FIG. 4. In contrast, in a casewhere the value represented by f4/f is −0.80 or less, the power ofconcave lens included in the fourth lens group 109 becomes weak, and itmay be difficult to give telecentricity desired for a long back focus.The value represented by f4/f is more preferably −0.60 or more and −0.30or less, and still more preferably −0.55 or more and −0.40 or less.

Further, in a case where the image-formation optical system 10 includesthe third lens group 107 described above, it is preferable that thesecond lens group 105 further satisfies a relationship represented bythe following expression (105), where a curvature radius at anobject-side surface of the lens located on most object side in the thirdlens group 107 is represented by R3 [mm].

0.85<R3/f   Expression (105)

The expression (105) is a conditional expression defining a curvature ofthe third lens group 107. The curvature of the third lens group 107 islargely related to spherical aberration of the image-formation opticalsystem 10 as a whole, and it is preferable to suppress the sphericalaberration as much as possible in the image-formation optical system 10as a whole. In a case where a value represented by R3/f is 0.85 or less,the spherical aberration of the image-formation optical system 10 as awhole becomes too large, which is not preferable. In contrast, althoughthe upper limit of the value represented by R3/f is not particularlydefined, it is preferably set to be less than 2.5. In a case where thevalue represented by R3/f is 2.5 or more, it may be difficult to selecta glass material to satisfy a desired focal length f3 of the third lensgroup 107.

Further, as schematically illustrated in FIG. 4, the image-formationoptical system 10 according to the present embodiment may furtherinclude a fifth lens group 111 having a negative refractive power at asubsequent stage (further image side) of the second lens group 105. Theadditional provision of such a fifth lens group 111 causes the opticalsystem to be telephoto, and makes it possible to reduce the size of theentire optical system without increasing a size of a stroke of a focus.

Further, the image-formation optical system 10 according to the presentembodiment preferably satisfies a condition represented by the followingexpression (106), where a focal length at a fluorescence wavelength ofinterest of the image-formation optical system 10 is represented byf(NIR) [mm], and a focal length at a visible light wavelength of theimage-formation optical system 10 is represented by f(V) [mm].

0.0025<(f(NIR)−f(V))/f(V)<0.0060   Expression (106)

The conditional expression represented by the expression (106) is aconditional expression that defines a range of an amount of misalignment(i.e., a range of axial chromatic aberration of the image-formationoptical system 10 as a whole) between a focal position of lightbelonging to the visible light wavelength band and a focal position oflight belonging to the near-infrared wavelength band. In a case wherethe amount of misalignment between the focal positions is 0.0025 orless, it is demanded to correct all chromatic aberration in the entirewavelength band from the visible light wavelength band to thenear-infrared wavelength band, and therefore, it may be difficult toselect a glass material. Further, attempting to achieve completechromatic aberration correction causes necessity to select a glassmaterial lacking in reliability in application to the medical field,which is not preferable. In contrast, in a case where the amount ofmisalignment between the focal positions is 0.0060 or more, chromaticaberration of the visible light wavelength band may be inadequatelycorrected, which is not preferable. The amount of misalignment betweenthe focus positions is more preferably 0.0030 or more and 0.0055 orless, and still more preferably 0.0040 or more and 0.0050 or less.

It is to be noted that the various lens characteristics of each lensgroup other than the above are not particularly limited, and may beappropriately set within the above conditional expressions so as tosatisfy a desired conditional expression for the image-formation opticalsystem 10 as a whole.

Further, as for the glass material of each lens included in theimage-formation optical system 10 according to the present embodiment,it is possible to use any glass material as long as it is a glassmaterial having high reliability in application to the medical field,and it is not particularly limited. However, it is preferable not to usea soft glass material which has large temperature variation and issusceptible to scratches, or a glass material which has a highrefractive index, does not easily transmit light of a low wavelength,and has an influence on color reproduction at the time of imaging.

The image-formation optical system 10 according to the presentembodiment has been described in detail above.

The rigid-scope optical system 1 according to the present the presentembodiment having the above-described configuration is able to achievereduction in size while ensuring reliability as a device, and to achievefurther resolution of a captured picture image to be obtained.

[Regarding Imaging Apparatus]

With the use of the rigid-scope optical system 1 described above, itbecomes possible to achieve an imaging apparatus (specifically, a camerahead unit (CHU)) applicable to various endoscope systems (e.g., rigidendoscope systems).

[Regarding Endoscope System]

Next, an endoscope system to which the rigid-scope optical system 1according to the present embodiment is applied will be briefly describedreferring to FIGS. 5 and 6. FIG. 5 is an explanatory diagramschematically illustrating an overall configuration of the endoscopesystem according to the present embodiment, and FIG. 6 is a blockdiagram illustrating an example of a hardware configuration of CCUincluded in the endoscope system according to the present embodiment.

As described above, it is possible to construct the endoscope system bycombining the rigid-scope optical system 1 (more specifically, theimaging apparatus including the rigid-scope optical system 1) and anendoscope unit (for example, a rigid-scope unit).

As schematically illustrated in FIG. 5, the endoscope system 500includes at least a rigid-scope unit 501, an imaging unit 503 having therigid-scope optical system 1, the visible light imaging device 3, andthe fluorescence imaging device 4 according to the present embodiment, acamera control unit (CCU) 505 that performs overall control on functionswhich the imaging unit 503 has, and a display apparatus 507.

Here, owing to the inclusion of the rigid-scope optical system 1described above, the imaging unit 503 including the rigid-scope opticalsystem 1 has similar effects as those achieved by the rigid-scopeoptical system 1, thus, the detailed description thereof will be omittedbelow.

The rigid-scope unit 501 serving as an example of the endoscope unitincludes, order from the object side (imaging target side), an objectivelens (not illustrated), multiple relay lenses (not illustrated), and aneyepiece (not illustrated). The objective lens forms an aerial image ofan imaging target, and the relay lenses performs relay image formationat unity magnification on the formed aerial image multiple times.Thereafter, the eyepiece performs afocal image formation on the lastaerial image, which makes it possible to observe the aerial image withthe naked eye.

Captured picture images (a visible light picture image and afluorescence picture image) generated in the imaging unit 503 areoutputted to the CCU 505, and the picture images are superimposed by theCCU 505, for example, to generate a superimposed picture image. Thecaptured visible light picture image and fluorescence picture image, andthe generated superimposed picture image are displayed on the displayapparatus 507 under the control of the CCU 505.

Here, the CCU 505 and the display apparatus 507 are not particularlylimited, and it is possible to use appropriately a known CCU and a knowndisplay apparatus.

[Regarding Hardware Configuration of CCU 505]

Next, referring to FIG. 6, a hardware configuration of the CCU 505according to an embodiment of the present disclosure will be describedin detail.

The mainly includes a CPU 901, a ROM 903, and a RAM 905. Further, theCCU 505 further includes a host bus 907, a bridge 909, an external bus911, an interface 913, an input device 915, an output device 917, astorage device 919, a drive 921, a connection port 923, and acommunication device 925.

The CPU 901 functions as an arithmetic processing unit and a controlunit, and controls an overall operation or a portion of operation in theCCU 505 in accordance with various programs recorded in the ROM 903, theRAM 905, the storage device 919, or a removable recoding medium 927. TheROM 903 stores a program, an arithmetic parameter, or the like to beused by the CPU 901. The RAM 905 primarily stores a program to be usedby CPU 901, a parameter that varies appropriately in executing theprogram, and the like. These are coupled to each other via the host bus907 including an internal bus such as a CPU bus.

The host bus 907 is coupled via the bridge 909 to the external bus 911such as a PCI (Peripheral Component Interconnect/Interface) bus.

The input device 915 is a manipulation unit manipulated by a user, suchas a mouse, a keyboard, a touch panel, a button, a switch, a lever, andthe like. Further, the input device 915 may be, for example, aremote-control unit (a so-called remote controller) using infra-red raysor other radio waves, or may be an external connection device 929 suchas a mobile telephone or a PDA compatible with manipulation of the CCU505. Further, the input device 915 includes, for example, an inputcontrol circuit that generates an input signal on the basis ofinformation inputted by the user using the above-described manipulationunit, and outputs the input signal to the CPU 901. The user of the CCU505 is able to input various types of data to the CCU 505 or provide theCCU 505 with an instruction on a processing operation by manipulatingthe input device 915.

The output device 917 includes a device that is able to visually oraudibly notifying the user of acquired information. Examples of such adevice include a display device such as a CRT display device, a liquidcrystal display device, a plasma display device, an EL display device,and a lamp, an audio output device such as a speaker or headphones, aprinter device, a mobile phone, a facsimile, and the like. The outputdevice 917 outputs, for example, a result obtained by various processesperformed by the CCU 505. Specifically, the display device displays theresult obtained by the various processes performed by the CCU 505 astext or an image. Meanwhile, the audio output device converts an audiosignal including reproduced audio data, acoustic data, and the like intoan analog signal, and outputs the analog signal.

The storage device 919 is a device for storing data configured as anexample of a storage of the CCU 505. The storage device 919 includes,for example, a magnetic storage device such as an HDD (Hard Disk Drive),a semiconductor storage device, an optical storage device, amagneto-optical storage device, or the like. The storage device 919stores a program to be executed by the CPU 901, various data, andvarious data acquired from the outside.

The drive 921 is a reader/writer for a recording medium, and is built inor externally attached to the CCU 505. The drive 921 reads informationrecorded on the removable recoding medium 927 such as a magnetic disk,an optical disc, a magneto-optical disk, or a semiconductor memory, andoutputs the read information to the RAM 905. In addition, the drive 921is also able to write a record to the removable recoding medium 927 suchas the magnetic disk, the optical disc, the magneto-optical disk, or thesemi-conductor memory that is mounted on the drive 921. The removablerecoding medium 927 may be, for example, a DVD medium, an HD-DVD medium,a Blu-ray (registered trademark) medium, etc. Further, the removablerecoding medium 927 may be compact flash (registered trademark)(CompactFlash: CF), a flash memory, an SD memory card (Secure Digitalmemory card), or the like. The removable recoding medium 927 may be, forexample, an IC card (Integrated Circuit card) on which a non-contacttype IC chip is mounted, an electronic device, or the like.

The connection port 923 is used to couple a device directly to the CCU505. Examples of the connection port 923 include a USB (Universal SerialBus) port, an IEEE1394 port, an SCSI (Small Computer System Interface)port, and the like. Other examples of the connection port 923 include anRS-232C port, an optical audio terminal, an HDMI (High-DefinitionMultimedia Interface) port, and the like. By coupling the externalconnection device 929 to the connection port 923, the CCU 505 acquiresvarious types of data directly from the external connection device 929or provide the external connection device 929 with various types ofdata.

The communication device 925 is, for example, a communication interfaceincluding a communication device to be coupled to a communicationnetwork 931. The communication device 925 may be, for example, acommunication card or the like for a wired or wireless LAN (Local AreaNetwork), Bluetooth (registered trademark), or WUSB (Wireless USB).Further, the communication device 925 may be a router for opticalcommunication, a router for ADSL (Asymmetric Digital Subscriber Line),or a modem for various types of communication. The communication device925 is able to transmit and receive a signal and the like to and fromthe Internet or another communication device in accordance with apredetermined protocol such as TCP/IP, for example. Further, thecommunication network 931 to be coupled to the communication device 925includes a network or the like coupled via wire or radio, and may be,for example, the Internet, a home LAN, infrared communication, radiowave communication, satellite communication, or the like.

An example of the hardware configuration that is able to achieve thefunctions of the CCU 505 according to an embodiment of the presentdisclosure has been described above. Each of the components describedabove may be configured using a general-purpose member, or may beconfigured by hardware specialized for the function of each component.Accordingly, the hardware configuration to be used is changeable asappropriate in accordance with the technical level at the time ofcarrying out the present embodiment.

Referring to FIGS. 5 and 6, the endoscope system 500 using therigid-scope optical system 1 according to the present embodiment hasbeen described briefly.

EXAMPLES

Hereinafter, the image-formation optical system included in therigid-scope optical system according to the present disclosure will bespecifically described with reference to the following Examples. It isto be noted that the following Examples are merely examples of theimage-formation optical system included in the rigid-scope opticalsystem according to the present disclosure, and the image-formationoptical system according to the present disclosure is not limited to theexamples indicated below.

In the following Examples, the image-formation optical system 10 ofExample 1 illustrated in FIG. 7A, the image-formation optical system 10of Example 2 illustrated in FIG. 7B, the image-formation optical system10 of Example 3 illustrated in FIG. 7C, the image-formation opticalsystem 10 of Example 4 illustrated in FIG. 7D, and the image-formationoptical system 10 of Example 5 illustrated in FIG. 7E are each used, anda commercially available lens design application (Code V manufactured bySynopsys, Inc.) is used, to perform a simulation on opticalcharacteristics of each image-formation optical system 10.

Here, in each of the following FIGS. 7A to 7E, a portion represented bya reference sign A corresponds to a portion in which a lensconfiguration corresponding to the color-separation prism 201 (e.g., thecolor-separation prism 201 illustrated in FIG. 3) according to thepresent disclosure is set, owing to necessity of condition setting forperforming the simulation.

Further, Examples 1 to 3 illustrated in FIGS. 7A to 7C are each anexample of the image-formation optical system 10 including the first tofourth lens groups, Example 4 illustrated in FIG. 7D is an example ofthe image-formation optical system 10 including the first to third lensgroups and the fifth lens group, and Example 5 illustrated in FIG. 7E isan example of the image-formation optical system 10 including the firstto third lens groups.

Hereinafter, a setting condition for each Example and obtainedsimulation results will be described in detail.

Example 1

The image-formation optical system 10 of Example 1 illustrated in FIG.7A is an image-formation optical system achieved by the first lens groupincluding three lenses, and the second to fourth lens groups eachincluding one lens.

Here, lens parameters of the respective lenses are as indicated in Table1 below.

TABLE 1 Example 1 R D index Abe 1 1 ∞ 0.700 1.76820 71.7991 1 2 ∞ 0.000Diaphragm 3 2 4 11.730 1.535 1.91048 31.3145 2 5 −68.406 2.109 3 6−17.344 0.700 1.73432 28.3200 3 7 8.460 2.132 4 8 −7.258 2.261 1.7044430.0500 5 9 20.948 2.800 1.73234 54.6727 5 10 −11.199 0.000 6 11 −55.7641.672 1.73234 54.6727 6 12 −17.236 2.353 7 13 21.247 2.113 1.6998055.4589 7 14 −47.976 3.750 8 15 ∞ 1.090 1.51500 85.6667 8 16 ∞ 0.700 917 ∞ 12.000 1.60718 38.0267 9 18 ∞ 0.930 10 19 ∞ 0.700 1.51872 64.166411 20 ∞ 0.500 1.51872 64.1664 11 21 ∞ 0.400 IMG 22 ∞

Further, values of the respective parameters of the expressions (101) to(106), which are achieved by such lens groups, are as indicated in Table2 below.

TABLE 2 Example 1 Entrance pupil diameter [mm] 4 Focal length f [mm]Furthest 16.60 1 m 16.49 Closest 16.32 F-number 4.1 Maximum image height2.9 L [mm] 26.28 Fb [mm] Furthest 14.19 1 m 14.76 Closest 15.61 f2 [mm]21.31 f4 [mm] −8.37 R3 11.73 Fb/f (Expression 101) Furthest 0.85 1 m0.90 Closest 0.96 L/f (Expression 102) 1.59 f2/f (Expression 103) 1.29f4/f (Expression 104) −0.51 R3/f (Expression 105) 0.71 Amount ofmisalignment (Expression 106) 0.0049

Further, obtained aberration diagrams are illustrated in FIGS. 8A to 8C.FIG. 8A is a longitudinal aberration diagram in the visible lightwavelength band of the image-formation optical system 10 of Example 1,FIG. 8B is a lateral aberration diagram in the visible light wavelengthband of the image-formation optical system 10 of Example 1, and FIG. 8Cis a longitudinal aberration diagram illustrating longitudinalaberration in the visible light wavelength band and longitudinalaberration in the fluorescence wavelength band together. Further, FIGS.8A and 8C each illustrate spherical aberration, field curvature, anddistortion aberration in this order from left side.

As is clear from the aberration diagrams illustrated in FIGS. 8A to 8C,the conditional expressions (101) to (106) according to the presentembodiment are each satisfied, and thus, it is appreciated that theimage-formation optical system 10 of Example 1 exhibits excellentcharacteristics for the spherical aberration, the field curvature, thedistortion aberration (FIGS. 8A and 8C) and also exhibits excellentcharacteristics for coma aberration (FIG. 8B).

Example 2

The image-formation optical system 10 of Example 2 illustrated in FIG.7B is an image-formation optical system achieved by the first lens groupincluding two lenses, and the second to fourth lens groups eachincluding one lens.

Here, lens parameters of the respective lenses are as indicated in Table3 below.

TABLE 3 Example 2 R D index Abe 1 1 ∞ 0.700 1.76820 71.7991 1 2 ∞ 0.000Diaphragm 3 0.780 2 4 12.383 1.651 1.91048 31.3145 2 5 −25.539 1.000 3 6−12.209 0.700 1.60718 38.0100 3 7 8.899 1.798 4 8 −9.308 2.600 1.8550523.7844 5 9 24.399 2.600 1.77621 49.6235 5 10 −9.542 2.354 6 11 22.0921.789 1.77621 49.6235 6 12 −49.937 3.735 7 13 ∞ 1.090 1.51500 85.6667 714 ∞ 0.700 8 15 ∞ 12.000 1.60718 38.0267 8 16 ∞ 0.930 9 17 ∞ 0.7001.51872 64.1664 10  18 ∞ 0.500 1.51872 64.1664 10  19 ∞ 0.400 IMG 20

Further, values of the respective parameters of the expressions (101) to(106), which are achieved by such lens groups, are as indicated in Table4 below.

TABLE 4 Example 2 Entrance pupil diameter [mm] 4 Focal length f [mm]Furthest 16.62 1 m 16.49 Closest 16.31 F-number 4.1 Maximum image height2.9 L [mm] 26.28 Fb [mm] Furthest 14.19 1 m 14.78 Closest 15.63 f2 [mm]19.95 f4 [mm] −7.66 R3 12.38 Fb/f (Expression 101) Furthest 0.85 1 m0.90 Closest 0.96 L/f (Expression 102) 1.59 f2/f (Expression 103) 1.21f4/f (Expression 104) −0.46 R3/f (Expression 105) 0.75 Amount ofmisalignment (Expression 106) 0.0050

Further, obtained aberration diagrams are illustrated in FIGS. 9A to 9C.FIG. 9A is a longitudinal aberration diagram in the visible lightwavelength band of the image-formation optical system 10 of Example 2,FIG. 9B is a lateral aberration diagram in the visible light wavelengthband of the image-formation optical system 10 of Example 2, and FIG. 9Cis a longitudinal aberration diagram illustrating longitudinalaberration in the visible light wavelength band and longitudinalaberration in the fluorescence wavelength band together. Further, FIGS.9A and 9C each illustrate spherical aberration, field curvature, anddistortion aberration in this order from left side.

As is clear from the aberration diagrams illustrated in FIGS. 9A to 9C,the conditional expressions (101) to (106) according to the presentembodiment are each satisfied, and thus, it is appreciated that theimage-formation optical system 10 of Example 2 exhibits excellentcharacteristics for the spherical aberration, the field curvature, thedistortion aberration (FIGS. 9A and 9C) and also exhibits excellentcharacteristics for coma aberration (FIG. 9B). However, it can beappreciated that the coma aberration and the field curvature are poor ascompared to the results of Example 1 illustrated in FIGS. 8A to 8C,because the number of lenses included in the first lens group 103 issmaller by one.

Example 3

The image-formation optical system 10 of Example 3 illustrated in FIG.7C is an image-formation optical system achieved by the first lens groupincluding three lenses, the second lens group and the third lens groupeach including one lens, and the fourth lens group including two lenses.

Here, lens parameters of the respective lenses are as indicated in Table5 below.

TABLE 5 Example 3 R D index Abe 1 1 ∞ 0.700 1.76820 71.7991 1 2 ∞ 0.000Diaphragm 3 0.780 2 4 1.500 1.59489 68.6233 2 5 23.760 0.100 3 6 32.4681.876 1.90614 37.3693 4 7 10.084 0.620 1.56252 42.6757 4 8 −110.3835.502 5 9 6.721 1.260 1.67066 29.6974 5 10 −6.366 0.669 6 11 61.7061.332 1.73234 54.6727 6 12 −12.501 0.100 7 13 −9.688 3.024 1.4984581.6072 7 14 24.026 2.742 8 15 −46.896 1.955 1.62032 63.3949 8 16 ∞3.923 9 17 ∞ 1.090 1.51872 64.1664 9 18 ∞ 0.700 10 19 ∞ 14.765 1.6071838.0267 10 20 ∞ 0.930 11 21 ∞ 0.700 1.51872 64.1664 12 22 ∞ 0.5001.51872 64.1664 13 23 ∞ 0.400 IMG 24

Further, values of the respective parameters of the expressions (101) to(106), which are achieved by such lens groups, are as indicated in Table6 below. It is to be noted that in this Example, it is not possible todefine the focal length f4, and the conditions represented by theexpression (104) is not satisfied.

TABLE 6 Example 3 Entrance pupil diameter [mm] 6 Focal length f [mm]Furthest 19.88 1 m 19.63 Closest 19.30 F-number 3.3 Maximum image height3.6 L [mm] 33.96 Fb [mm] Furthest 15.32 1 m 16.21 Closest 17.45 f2 [mm]25.88 f4 [mm] — R3 23.76 Fb/f (Expression 101) Furthest 0.77 1 m 0.83Closest 0.90 L/f (Expression 102) 1.73 f2/f (Expression 103) 1.32 f4/f(Expression 104) — R3/f (Expression 105) 1.21 Amount of misalignment(Expression 106) 0.0037

Obtained aberration diagrams are illustrated in FIGS. 10A to 10C. FIG.10A is a longitudinal aberration diagram in the visible light wavelengthband of the image-formation optical system 10 of Example 3, FIG. 10B isa lateral aberration diagram in the visible light wavelength band of theimage-formation optical system 10 of Example 3, and FIG. 10C is alongitudinal aberration diagram illustrating longitudinal aberration inthe visible light wavelength band and longitudinal aberration in thefluorescence wavelength band together. Further, FIGS. 10A and 10C eachillustrate spherical aberration, field curvature, and distortionaberration in this order from left side.

As is clear from the aberration diagrams illustrated in FIGS. 10A to10C, the conditional expressions (101) to (106) according to the presentembodiment are each satisfied, and thus, it is appreciated that theimage-formation optical system 10 of Example 3 exhibits excellentcharacteristics for the spherical aberration, the field curvature, thedistortion aberration (FIGS. 10A and 10C) and also exhibits excellentcharacteristics for coma aberration (FIG. 10B).

It is to be noted that Example 3 is an example in which the entrancepupil diameter is increased and the F-number is decreased. The reductionin F-number makes it possible to increase the resolution limit, and thisis an effective example in a case where high resolution is demanded inthe future.

Example 4

The image-formation optical system 10 of Example 4 illustrated in FIG.7D is an image-formation optical system achieved by the first lens groupincluding two lenses, the second lens group and the third lens groupeach including one lens, and the fifth lens group including one lens.

Here, lens parameters of the respective lenses are as indicated in Table7 below.

TABLE 7 Example 4 R D index Abe 1 1 ∞ 0.700 1.76820 71.7991 1 2 ∞ 0.000Diaphragm 3 0.780 2 4 35.969 1.543 1.88814 40.7993 2 5 −27.606 2.843 3 6−5.049 0.700 1.78415 40.8809 3 7 22.028 0.214 4 8 −17.878 2.579 1.5948868.6290 4 9 −5.285 1.338 5 10 14.584 2.324 1.59488 68.6290 5 11 −9.1242.166 6 12 −9.284 0.600 1.62409 36.2994 6 13 ∞ 2.500 7 14 ∞ 1.0901.51872 64.1664 7 15 ∞ 0.700 8 16 ∞ 12.000 1.60718 38.0267 8 17 ∞ 0.9309 18 ∞ 0.700 1.51872 64.1664 10  19 ∞ 0.500 1.51872 64.1664 10  20 ∞0.400 IMG 21

Further, values of the respective parameters of the expressions (101) to(106), which are achieved by such lens groups, are as indicated in Table8 below. It is to be noted that in this Example, the focal length f4 isdefined as in Table 10 below, and as a result, the condition representedby the expression (104) is not satisfied.

TABLE 8 Example 4 Entrance pupil diameter [mm] 2.5 Focal length f [mm]Furthest 17.05 1 m 16.61 Closest 15.78 F-number 6.6 Maximum image height2.4 L [mm] 27.29 Fb [mm] Furthest 13.52 1 m 13.52 Closest 13.52 f2 [mm]9.79 f4 [mm] −5.18 R3 35.97 Fb/f (Expression 101) Furthest 0.79 1 m 0.81Closest 0.86 L/f (Expression 102) 1.64 f2/f (Expression 103) 0.59 f4/f(Expression 104) −0.31 R3/f (Expression 105) 2.17 Amount of misalignment(Expression 106) 0.0047

Obtained aberration diagrams are illustrated in FIGS. 11A to 11C. FIG.11A is a longitudinal aberration diagram in the visible light wavelengthband of the image-formation optical system 10 of Example 4, FIG. 11B isa lateral aberration diagram in the visible light wavelength band of theimage-formation optical system 10 of Example 4, and FIG. 11C is alongitudinal aberration diagram illustrating longitudinal aberration inthe visible light wavelength band and longitudinal aberration in thefluorescence wavelength band together. Further, FIGS. 11A and 11C eachillustrate spherical aberration, field curvature, and distortionaberration in this order from left side.

As is clear from the aberration diagrams illustrated in FIGS. 11A to11C, the conditional expressions (101) to (103) and the conditionalexpressions (105) to (106) according to the present embodiment are eachsatisfied, and thus, it is appreciated that the image-formation opticalsystem 10 of Example 4 exhibits excellent characteristics for thespherical aberration, the field curvature, the distortion aberration(FIGS. 11A and 11C) and also exhibits excellent characteristics for comaaberration (FIG. 11B).

It is to be noted that Example 4 is an example in which the entrancepupil diameter is decreased and compatibility is limited only to a rigidscope having a relatively small diameter. The limitation in thecompatible rigid scope lowers degree of design difficulty, and makesaberration correction easier while reducing the number of lenses.

Example 5

The image-formation optical system 10 of Example 5 illustrated in FIG.7E is an image-formation optical system achieved by the first lens groupincluding two lenses, and the second lens group and the third lens groupeach including one lens.

Here, lens parameters of the respective lenses are as indicated in Table9 below.

TABLE 9 Example 5 R D index Abe 1 1 ∞ 0.700 1.76820 71.7991 1 2 ∞ 0.000Diaphragm 3 0.780 2 4 13.952 1.545 1.96073 32.3194 2 5 −113.878 2.722 36 −13.480 0.700 1.83348 23.5319 3 7 13.545 1.036 4 8 −9.166 2.8001.62286 60.3227 4 9 −7.198 2.000 5 10 19.581 1.789 1.62033 63.3990 5 11−84.372 4.772 6 12 ∞ 1.090 1.51872 64.1664 6 13 ∞ 0.700 7 14 ∞ 17.6241.60718 38.0267 7 15 ∞ 0.930 8 16 ∞ 0.700 1.51872 64.1664 9 17 ∞ 0.5001.51872 64.1664 9 18 ∞ 0.400 IMG 19

Further, values of the respective parameters of the expressions (101) to(106), which are achieved by such lens groups, are as indicated in Table10 below. It is to be noted that in this Example, the air-equivalentoptical path length L from the image-formation optical system 10 to thecolor-separation-prism optical system is defined as in Table 12, and asa result, the condition represented by the expression (102) is notsatisfied. In addition, in this Example, the focal length f4 is definedas in Table 12 below, and as a result, the condition represented by theexpression (104) is not satisfied.

TABLE 10 Example 5 Entrance pupil diameter [mm] 4 Focal length f [mm]Furthest 24.56 1 m 24.27 Closest 23.85 F-number 6.1 Maximum image height4.3 L [mm] 32.15 Fb [mm] Furthest 18.03 1 m 19.29 Closest 21.21 f2 [mm]25.79 f4 [mm] −8.01 R3 13.95 Fb/f (Expression 101) Furthest 0.73 1 m0.79 Closest 0.89 L/f (Expression 102) 1.32 f2/f (Expression 103) 1.06f4/f (Expression 104) −0.33 R3/f (Expression 105) 0.57 Amount ofmisalignment (Expression 106) 0.0048

Obtained aberration diagrams are illustrated in FIGS. 12A to 12C. FIG.12A is a longitudinal aberration diagram in the visible light wavelengthband of the image-formation optical system 10 of Example 5, FIG. 12B isa lateral aberration diagram in the visible light wavelength band of theimage-formation optical system 10 of Example 5, and FIG. 12C is alongitudinal aberration diagram illustrating longitudinal aberration inthe visible light wavelength band and longitudinal aberration in thefluorescence wavelength band together. Further, FIGS. 12A and 12C eachillustrate spherical aberration, field curvature, and distortionaberration in this order from left side.

As is clear from the aberration diagrams illustrated in FIGS. 12A to12C, the conditional expressions (101) to (103) and the conditionalexpressions (105) to (106) according to the present embodiment are eachsatisfied, and thus, it is appreciated that the image-formation opticalsystem 10 of Example 5 exhibits excellent characteristics for thespherical aberration, the field curvature, the distortion aberration(FIGS. 12A and 12C) and also exhibits excellent characteristics for comaaberration (FIG. 12B).

It is to be noted that such an Example is a case in which a size of theimaging device is increased. Although the entrance pupil diameter is thesame as Example 1 or the like, the increase in focal length forobtaining the same angle of view makes it possible to increase theF-number. For this reason, the spherical aberration rarely occurs, andit becomes possible to achieve a design with a small number of lenses.

A preferred embodiment of the present disclosure has/have been describedabove in detail with reference to the accompanying drawings, but thetechnical scope of the present disclosure is not limited to such anembodiment. It is apparent that a person having ordinary skill in theart of the present disclosure can arrive at various alterations andmodifications within the scope of the technical idea described in theappended claims, and it is understood that such alterations andmodifications naturally fall within the technical scope of the presentdisclosure.

Furthermore, the effects described herein are merely illustrative andexemplary, and not limiting. That is, the technique according to thepresent disclosure can exert other effects that are apparent to thoseskilled in the art from the description herein, in addition to theabove-described effects or in place of the above-described effects.

It is to be noted that the following configurations also belong to thetechnical scope of the present disclosure.

(1)

A rigid-scope optical system including:

an image-formation optical system that causes an image in each ofwavelength bands to be formed in a predetermined imaging device, thewavelength bands including a fluorescence wavelength band belonging to anear-infrared light wavelength band and a visible light wavelength band;and

a color-separation-prism optical system having a dichroic film thatseparates an optical path of light to be imaged by the image-formationoptical system into an optical path of the visible light wavelength bandand an optical path of the fluorescence wavelength band, in which

the image-formation optical system causes the respective images to beformed in a fluorescence imaging device and a visible light imagingdevice, the fluorescence imaging device and the visible light imagingdevice being disposed to cause an amount of misalignment between afluorescence image formation position and a visible light imageformation position caused by the image-formation optical system tocorrespond to a difference between an optical path length offluorescence and an optical path length of visible light, thefluorescence and the visible light forming the respective images via thecolor-separation-prism optical system, and,

where a focal length of the image-formation optical system isrepresented by f [mm], and an air-equivalent optical path length fromthe image-formation optical system to an imaging device is representedby Fb [mm], the image-formation optical system has the focal length andthe air-equivalent optical path length that satisfy a conditionrepresented by the following expression (1),

Fb/f>0.72   expression (1).

(2)

The rigid-scope optical system according to (1), in which

the image-formation optical system includes, in order from object sideto image side,

-   -   at least a diaphragm, a first lens group having a positive        refractive power, and a second lens group having a positive        refractive power,    -   the first lens group including, in order from the object side to        the image side, a lens having a negative refractive power with a        concave surface facing the object side, and at least one lens        having a positive refractive power,    -   the second lens group being a focus group that performs focusing        depending on an object distance, and

the image-formation optical system satisfies a condition represented bythe following expression (2), where an air-equivalent optical pathlength from the image-formation optical system to thecolor-separation-prism optical system is represented by L [mm],

1.4<L/f<1.8   expression (2).

(3)

The rigid-scope optical system according to (2), in which

the image-formation optical system further includes, between thediaphragm and the first lens group, in order from the object side to theimage side, at least one of a third lens group having a positiverefractive power or a fourth lens group having a negative refractivepower, and

the fourth lens group satisfies a condition represented by the followingexpression (3), where a focal length of the fourth lens group in theimage-formation optical system is represented by f4 [mm],

−0.80<f4/f<−0.35   expression (3)

(4)

The rigid-scope optical system according to (3), in which

the image-formation optical system includes the third lens group, and

the second lens group further satisfies a relationship represented bythe following expression (4), where a curvature radius at an object-sidesurface of a lens located on most object side in the third lens group isrepresented by R3 [mm],

0.85<R3/f   expression (4).

(5)

The rigid-scope optical system according to any one of (2) to (4), inwhich

the second lens group in the image-formation optical system furthersatisfies a condition represented by the following expression (5), wherea focal length of the second lens group is represented by f2 [mm],

1.0<f2/f<1.4   expression (5)

(6)

The rigid-scope optical system according to any one of (2) to (5), inwhich the image-formation optical system further includes a fifth lensgroup having a negative refractive power at a subsequent stage of thesecond lens group.

(7)

The rigid-scope optical system according to any one of (1) to (6), inwhich the image-formation optical system satisfies a conditionrepresented by the following expression (6), where a focal length at afluorescence wavelength of the image-formation optical system isrepresented by f(NIR) [mm], and a focal length at a visible lightwavelength of the image-formation optical system is represented by f(V)[mm],

0.0025<(f(NIR)−f(V))/f(V)<0.0060   expression (6)

(8)

The rigid-scope optical system according to (7), in which

the visible light imaging device is fixed at a position on animage-formation surface of the image-formation optical system, and

the fluorescence imaging device is fixed at a position in which anoptical path difference satisfies the expression (6).

(9)

The rigid-scope optical system according to (7), in which

the visible light imaging device is fixed at a position on animage-formation surface of the image-formation optical system,

the fluorescence imaging device is provided to allow a separationdistance from the color-separation-prism optical system to be variable,and

the rigid-scope optical system further includes a fluorescence pictureimage focusing mechanism that varies an optical path difference to causethe expression (6) to be satisfied.

(10)

The rigid-scope optical system according to any one of (1) to (9), inwhich the color-separation-prism optical system includes

a color-separating prism including the dichroic film, and

a bandpass filter provided between the color-separating prism and thefluorescence imaging device, the bandpass filter having an entrancesurface perpendicular to an optical axis.

(11)

The rigid-scope optical system according to (10), in which

the dichroic film has a transmittance of 90% or more in a wavelengthband of 780 to 880 nm, and a transmittance of 10% or less in awavelength band of 400 to 720 nm, and

the bandpass filter has a transmittance of 90% or more in a wavelengthband of 813 to 850 nm, and a transmittance of 10% or less in awavelength band of 350 to 805 nm.

(12)

An imaging apparatus including a rigid-scope optical system, therigid-scope optical system including

an image-formation optical system that causes an image in each ofwavelength bands to be formed in a predetermined imaging device, thewavelength bands including a fluorescence wavelength band belonging to anear-infrared light wavelength band and a visible light wavelength band,

a color-separation-prism optical system having a dichroic film thatseparates an optical path of light to be imaged by the image-formationoptical system into an optical path of the visible light wavelength bandand an optical path of the fluorescence wavelength band,

a visible light imaging device that forms an image of the visible lightwavelength band, and

a fluorescence imaging device that forms an image in the fluorescencewavelength band, in which

the visible light imaging device and the fluorescence imaging device aredisposed to cause an optical path difference between an optical pathlength of a visible light wavelength band and an optical path length ofa fluorescence wavelength band to correspond to an amount ofmisalignment between a fluorescence image formation position and avisible light image formation position caused by the image-formationoptical system, the visible light forming an image in the visible lightimaging device via the color-separation-prism optical system, thefluorescence forming an image in the fluorescence imaging device via thecolor-separation-prism optical system, and,

where a focal length of the image-formation optical system isrepresented by f [mm], and an air-equivalent optical path length fromthe image-formation optical system to an imaging device is representedby Fb [mm], the image-formation optical system has the focal length andthe air-equivalent optical path length that satisfy a conditionrepresented by the following expression (1),

Fb/f>0.72   expression (1).

(13)

An endoscope unit including:

a rigid-scope unit that generates an image of a predetermined imagingtarget of a fluorescence wavelength band belonging to a near-infraredlight wavelength band and an image of the predetermined imaging targetof a visible light wavelength band;

an imaging unit that includes a rigid-scope optical system coupled tothe rigid-scope unit, a visible light imaging device in which the imageof the visible light wavelength band is formed, and a fluorescenceimaging device in which the image of the fluorescence wavelength band isformed, and generates a captured picture image of the imaging target ofthe fluorescence wavelength band and a captured picture image of theimaging target of the visible light wavelength band, in which

the rigid-scope optical system includes

-   -   an image-formation optical system that causes an image in each        of the fluorescence wavelength band and the visible light        wavelength band to be formed in a predetermined imaging device,        and    -   a color-separation-prism optical system having a dichroic film        that separates an optical path of light to be imaged by the        image-formation optical system into an optical path of the        visible light wavelength band and an optical path of the        fluorescence wavelength band,

the visible light imaging device and the fluorescence imaging device aredisposed to cause an optical path difference between an optical pathlength of a visible light wavelength band and an optical path length ofa fluorescence wavelength band to correspond to an amount ofmisalignment between a fluorescence image formation position and avisible light image formation position caused by the image-formationoptical system, the visible light forming an image in the visible lightimaging device via the color-separation-prism optical system, thefluorescence forming an image in the fluorescence imaging device via thecolor-separation-prism optical system, and,

where a focal length of the image-formation optical system isrepresented by f [mm], and an air-equivalent optical path length fromthe image-formation optical system to an imaging device is representedby Fb [mm], the image-formation optical system has the focal length andthe air-equivalent optical path length that satisfy a conditionrepresented by the following expression (1),

Fb/f>0.72   expression (1).

REFERENCE SIGNS LIST

-   1 rigid-scope optical system-   3 visible light imaging device-   4 fluorescence imaging device-   10 image-formation optical system-   20 color-separation-prism optical system-   30 fluorescence picture image focusing mechanism-   101 diaphragm-   103 first lens group-   105 second lens group-   107 third lens group-   109 fourth lens group-   111 fifth lens group-   201 color-separation prism-   203 dichroic film-   211 first prism-   213 second prism-   215 narrow-band bandpass filter-   217 infrared cut filter-   500 endoscope system-   501 rigid-scope unit-   503 imaging unit-   505 CCU-   507 display apparatus

1. A rigid-scope optical system comprising: an image-formation opticalsystem that causes an image in each of wavelength bands to be formed ina predetermined imaging device, the wavelength bands including afluorescence wavelength band belonging to a near-infrared lightwavelength band and a visible light wavelength band; and acolor-separation-prism optical system having a dichroic film thatseparates an optical path of light to be imaged by the image-formationoptical system into an optical path of the visible light wavelength bandand an optical path of the fluorescence wavelength band, wherein theimage-formation optical system causes the respective images to be formedin a fluorescence imaging device and a visible light imaging device, thefluorescence imaging device and the visible light imaging device beingdisposed to cause an amount of misalignment between a fluorescence imageformation position and a visible light image formation position causedby the image-formation optical system to correspond to a differencebetween an optical path length of fluorescence and an optical pathlength of visible light, the fluorescence and the visible light formingthe respective images via the color-separation-prism optical system,and, where a focal length of the image-formation optical system isrepresented by f [mm], and an air-equivalent optical path length fromthe image-formation optical system to an imaging device is representedby Fb [mm], the image-formation optical system has the focal length andthe air-equivalent optical path length that satisfy a conditionrepresented by the following expression (1),Fb/f>0.72   expression (1).
 2. The rigid-scope optical system accordingto claim 1, wherein the image-formation optical system includes, inorder from object side to image side, at least a diaphragm, a first lensgroup having a positive refractive power, and a second lens group havinga positive refractive power, the first lens group including, in orderfrom the object side to the image side, a lens having a negativerefractive power with a concave surface facing the object side, and atleast one lens having a positive refractive power, the second lens groupbeing a focus group that performs focusing depending on an objectdistance, and the image-formation optical system satisfies a conditionrepresented by the following expression (2), where an air-equivalentoptical path length from the image-formation optical system to thecolor-separation-prism optical system is represented by L [mm],1.4<L/f<1.8   expression (2).
 3. The rigid-scope optical systemaccording to claim 2, wherein the image-formation optical system furtherincludes, between the diaphragm and the first lens group, in order fromthe object side to the image side, at least one of a third lens grouphaving a positive refractive power or a fourth lens group having anegative refractive power, and the fourth lens group satisfies acondition represented by the following expression (3), where a focallength of the fourth lens group in the image-formation optical system isrepresented by f4 [mm],−0.80<f4/f<−0.35   expression (3)
 4. The rigid-scope optical systemaccording to claim 3, wherein the image-formation optical systemincludes the third lens group, and the second lens group furthersatisfies a relationship represented by the following expression (4),where a curvature radius at an object-side surface of a lens located onmost object side in the third lens group is represented by R3 [mm],0.85<R3/f   expression (4).
 5. The rigid-scope optical system accordingto claim 2, wherein the second lens group in the image-formation opticalsystem further satisfies a condition represented by the followingexpression (5), where a focal length of the second lens group isrepresented by f2 [mm],1.0<f2/f<1.4   expression (5)
 6. The rigid-scope optical systemaccording to claim 2, wherein the image-formation optical system furtherincludes a fifth lens group having a negative refractive power at asubsequent stage of the second lens group.
 7. The rigid-scope opticalsystem according to claim 1, wherein the image-formation optical systemsatisfies a condition represented by the following expression (6), wherea focal length at a fluorescence wavelength of the image-formationoptical system is represented by f(NIR) [mm], and a focal length at avisible light wavelength of the image-formation optical system isrepresented by f(V) [mm],0.0025<(f(NIR)−f(V))/f(V)<0.0060   expression (6)
 8. The rigid-scopeoptical system according to claim 7, wherein the visible light imagingdevice is fixed at a position on an image-formation surface of theimage-formation optical system, and the fluorescence imaging device isfixed at a position in which an optical path difference satisfies theexpression (6).
 9. The rigid-scope optical system according to claim 7,wherein the visible light imaging device is fixed at a position on animage-formation surface of the image-formation optical system, thefluorescence imaging device is provided to allow a separation distancefrom the color-separation-prism optical system to be variable, and therigid-scope optical system further includes a fluorescence picture imagefocusing mechanism that varies an optical path difference to cause theexpression (6) to be satisfied.
 10. The rigid-scope optical systemaccording to claim 1, wherein the color-separation-prism optical systemincludes a color-separating prism including the dichroic film, and abandpass filter provided between the color-separating prism and thefluorescence imaging device, the bandpass filter having an entrancesurface perpendicular to an optical axis.
 11. The rigid-scope opticalsystem according to claim 10, wherein the dichroic film has atransmittance of 90% or more in a wavelength band of 780 to 880 nm, anda transmittance of 10% or less in a wavelength band of 400 to 720 nm,and the bandpass filter has a transmittance of 90% or more in awavelength band of 813 to 850 nm, and a transmittance of 10% or less ina wavelength band of 350 to 805 nm.
 12. An imaging apparatus comprisinga rigid-scope optical system, the rigid-scope optical system includingan image-formation optical system that causes an image in each ofwavelength bands to be formed in a predetermined imaging device, thewavelength bands including a fluorescence wavelength band belonging to anear-infrared light wavelength band and a visible light wavelength band,a color-separation-prism optical system having a dichroic film thatseparates an optical path of light to be imaged by the image-formationoptical system into an optical path of the visible light wavelength bandand an optical path of the fluorescence wavelength band, a visible lightimaging device that forms an image of the visible light wavelength band,and a fluorescence imaging device that forms an image in thefluorescence wavelength band, wherein the visible light imaging deviceand the fluorescence imaging device are disposed to cause an opticalpath difference between an optical path length of an optical path forvisible light and an optical path length of an optical path forfluorescence to correspond to an amount of misalignment between afluorescence image formation position and a visible light imageformation position caused by the image-formation optical system, thevisible light forming an image in the visible light imaging device viathe color-separation-prism optical system, the fluorescence forming animage in the fluorescence imaging device via the color-separation-prismoptical system, and, where a focal length of the image-formation opticalsystem is represented by f [mm], and an air-equivalent optical pathlength from the image-formation optical system to an imaging device isrepresented by Fb [mm], the image-formation optical system has the focallength and the air-equivalent optical path length that satisfy acondition represented by the following expression (1),Fb/f>0.72   expression (1).
 13. An endoscope unit comprising: arigid-scope unit that generates an image of a predetermined imagingtarget of a fluorescence wavelength band belonging to a near-infraredlight wavelength band and an image of the predetermined imaging targetof a visible light wavelength band; an imaging unit that includes arigid-scope optical system coupled to the rigid-scope unit, a visiblelight imaging device in which the image of the visible light wavelengthband is formed, and a fluorescence imaging device in which the image ofthe fluorescence wavelength band is formed, and generates a capturedpicture image of the imaging target of the fluorescence wavelength bandand a captured picture image of the imaging target of the visible lightwavelength band, wherein the rigid-scope optical system includes animage-formation optical system that causes an image in each of thefluorescence wavelength band and the visible light wavelength band to beformed in a predetermined imaging device, and a color-separation-prismoptical system having a dichroic film that separates an optical path oflight to be imaged by the image-formation optical system into an opticalpath of the visible light wavelength band and an optical path of thefluorescence wavelength band, the visible light imaging device and thefluorescence imaging device are disposed to cause an optical pathdifference between an optical path length of an optical path for visiblelight and an optical path length of an optical path for fluorescence tocorrespond to an amount of misalignment between a fluorescence imageformation position and a visible light image formation position causedby the image-formation optical system, the visible light forming animage in the visible light imaging device via the color-separation-prismoptical system, the fluorescence forming an image in the fluorescenceimaging device via the color-separation-prism optical system, and, wherea focal length of the image-formation optical system is represented by f[mm], and an air-equivalent optical path length from the image-formationoptical system to an imaging device is represented by Fb [mm], theimage-formation optical system has the focal length and theair-equivalent optical path length that satisfy a condition representedby the following expression (1),Fb/f>0.72   expression (1).